The present invention relates to techniques for recording digital information on tape.
There are various applications in which it would be helpful to provide a digital track along side an analog track. For example, a system with a facility for carrying on a "conversation" with a person is particularly useful for telephonic banking. In such application, the cost of the system must be justified. Yet, it must also be very reliable--particularly in such operations as banking transactions.
In carrying on a conversation, the system must be able to give different vocalizations in response to various conditions. If the system asks a question to the banking customer and receives an answer, the system should determine whether the transaction is complete, and if not, ask the next intelligent question. However, if the response from the customer is inaudible or subject to some other infirmity, the system must remedy this problem by reasking the question or by advising the customer of the nature of the problem and then asking him to repeat his response. Hence, the system must have a facility for determining whether the "conversation" is proceeding properly, and must further be able to give vocalizations, whether in the form of responses or questions, depending on various conditions.
Such a system may use tape recorders to carry on and record the "conversation." The system is faced with the problem of keeping an accurate record of where various responses or questions or other vocalizations are prerecorded on a tape. For this, the system can use two tracks of recording, one track having the audio vocalizations prerecorded thereon, and the other track having digital data recorded thereon which corresponds to the audio information. The system can select a vocalization to articulate by playing back the tape recorder and by means of the digital data recorded on the tape, can move the tape to the proper starting point for the selected vocalization.
Although audio tape recorders are well adapted for recording signals such as voice that have a zero DC component, they are not well-suited to recording digital data. Digital data can easily include a substantial DC component, such as a string of high logic signals which will degrade to a zero or middle level if it is presented to the audio tape recorder for recording. This results in a loss of data.
The simplest, least expensive way to write digital data on an audio tape recorder requires a startup period. Basically, a bit coming off a processor, port can be coupled to the audio input of the recorder. The computer or processor can use a modulation scheme using square waves to signify the logic states. However, when the processor stops modulating the square wave signal, its voltage level is at one of the two extremes, high or low. This causes the electronics of the tape recorder to drift up to or down to that level. The, when new information is applied to the recorder for recording, the recorder electronics requires between one-half and one second to recover the reference or datum level. In other words, the reference level is lost at the termination of an ordinary square wave and unless it is restored by a subsequent procedure, the next data that are written will not be reliably recorded. In various applications where a given response is needed from the system, such as where the system "converses" with a person, the time required for a subsequent proceeding is not available.
It is therefore another object of the present invention to provide a method and apparatus for reliably recording digital data using an audio tape recorder.
It is further object to provide an inexpensive method and apparatus for recording digital data with an audio tape recorder.